Entropy‐stabilized oxides (ESOs) provide an alternative route to novel materials discovery and synthesis. It is, however, a challenge to demonstrate that the constituent elements in an entropy‐stabilized crystal are homogeneously and randomly dispersed among a particular sublattice, resulting in a true solid solution with no evidence of local order or clustering. In this work, we present the application and analysis of extended X‐ray absorption fine structure (EXAFS) on the prototype ESO composition MgxNixCoxCuxZnxO (x=0.2). In so doing, we can quantify the local atomic structure on an element‐by‐element basis. We conclude that local bond lengths between metal and oxygen vary around each absorbing cation, with notable distortion around the Cu–O polyhedra. By the second near neighbor (i.e., the cation‐cation pair), interatomic distances are uniform to the extent that the collected data can resolve. Crystal models that best fit the experimental scattering data include cations that are distributed randomly on an FCC sublattice with minimal positional disorder, with an interleaved FCC anion sublattice with oxygen ions displaced from the ideal locations to accommodate the distortions in the cation polyhedra. Density functional theory calculations of the ESO system yield a significant broadening in the positional distribution for the oxygen sublattice compared to that for the cation sublattice for all peaks, showing consistency with the conclusion from the experimental data that the distortion from an ideal rock salt structure occurs primarily through disorder in the oxygen sublattice.

Density functional theory calculations were carried out for three entropic rocksalt oxides, (Mg0.1Co0.1Ni0.1Cu0.1Zn0.1)O0.5, termed J14, and J14 + Li and J14 + Sc, to understand the role of charge neutrality and electronic states on their properties, and to probe whether simple expressions may exist that predict stability. The calculations predict that the average lattice constants of the ternary structures provide good approximations to that of the random structures. For J14, Bader charges are transferable between the binary, ternary, and random structures. For J14 + Sc and J14 + Li, average Bader charges in the entropic structures can be estimated from the ternary compositions. Addition of Sc to J14 reduces the majority of Cu, which show large displacements from ideal lattice sites, along with reduction of a few Co and Ni cations. Addition of Li to J14 reduces the lattice constant, consistent with experiment, and oxidizes some of Co as well as some of Ni and Cu. The Bader charges and spin-resolved density of states (DOS) for Co+3 in J14 + Li are very different from Co+2, while for Cu and Ni the Bader charges form continuous distributions and the two DOS are similar for the two oxidation states. Experimental detection of different oxidation states may therefore be challenging for Cu and Ni compared to Co. Based on these results, empirical stability parameters for these entropic oxides may be more complicated than those for non-oxide entropic solids.

A set of effective chemical potentials (ECPs) are derived that connect energies of (Co, Fe, Ni, Zn)Fe2O4 spinels and oxides calculated at 0 K from density functional theory (DFT) to free energies in high temperature and pressure water. The ECPs are derived and validated by solving a system of linear equations that combine DFT and experimental free energies for NiO, ZnO, Fe2O3, Fe3O4, FeO(OH), CoFe2O4, ZnFe2O4, NiFe2O4 and H2O. To connect to solution phase chemistry, a set of ECPs are also derived for solvated Ni(2+), Zn(2+), Fe(2+) and Fe(3+) ions using an analogous set of linear equations and the solid ECPs. The ECPs are used to calculate free energies of low index stoichiometric surfaces of nickel oxide (NiO) and nickel ferrite (NiFe2O4) in water as a function of temperature from 300 to 600 K at a pressure of 155 bar. Surface denuding at high temperatures is predicted, the implications of which for the formation of oxide corrosion products on heat transfer surfaces in light-water nuclear reactors are discussed.

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We have studied the nature of the surface charge distribution in CeTe3. This is a simple, cleavable, layered material with a robust one-dimensional incommensurate charge density wave (CDW). Scanning tunneling microscopy (STM) has been applied on the exposed surface of a cleaved single crystal. At 77 K, the STM images show both the atomic lattice of surface Te atoms arranged in a square net and the CDW modulations oriented at 45• with respect to the Te net. Fourier transform of the STM data shows Te square lattice peaks, and peaks related to the CDW oriented at 45• to the lattice peaks. In addition, clear peaks are present, consistent with subsurface structure and wave vector mixing effects. These data are supported by electronic structure calculations, which show that the subsurface signal most likely arises from a lattice of Ce atoms situated 2.53Å below the surface Te net.

The thermodynamic stability of Th-doped yttrium iron garnet (Y3Fe5O12, YIG) as a possible actinide-bearing material has been investigated using calorimetric measurements and first-principles electronic-structure calculations.

A comprehensive theoretical investigation of nickel ferrite (NiFe 2 O 4 ) surfaces is undertaken to understand the structure and stability of nanocrystallites that would be present under conditions of hydrothermal synthesis (HTS). In particular, the focus is on conditions characteristic to an operating pressurized water nuclear reactor (PWR). Solid− liquid equilibrium is assumed between bulk nickel ferrite and the aqueous environment saturated with respect to nickel ferrite. A theoretical framework is developed in which the surface energies are evaluated in terms of concentrations of aqueous metal cations, pH, temperature, and pressure. The energies of the bare and water terminated surfaces are calculated and discussed. Surfaces that have more metal cations exposed are found to be more stable. Water adsorption on the nickel ferrite surfaces is an exothermic process, with the magnitude of exothermicity decreasing as a function of temperature. At temperatures relevant to operating PWRs, the energy gain due to water adsorption is negligible. The most stable surfaces are along the (111) planes and are predicted to have negative surface energies. This indicates that, in an operating PWR, nickel ferrite tends to increase its surface area, giving rise to a highly porous thermodynamic ground state. This provides an explanation for the porous nature of the nickel ferrite deposits observed to form on PWR fuel rods.

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